1. Field of the Invention
The invention relates generally to the production of consolidated cellulosic products and, more specifically, to the production of a consolidated cellulosic product using a steam injection pressing operation wherein steam is injected and exhausted through a single platen, while achieving excellent steam distribution throughout the cellulosic product being consolidated so that an opposite platen can be textured or embossed.
2. Brief Description of Related Technology
Synthetic resin adhesives, such as phenol-based resins, are widely used as binders in the manufacture of composite articles, such as consolidated cellulosic products (e.g., waferboard, chipboard, oriented strandboard, or fiberboard). Such composites may be formed by various processes and may be formed in a variety of desired shapes and sizes depending on the intended end use of the composites. Generally, however, consolidated cellulosic products are formed by combining a phenolic resin, such as a phenol-formaldehyde resin, with filler material, such as cellulosic fibers or cellulosic particles, and then bonding the filler material together in a press that provides heat and pressure. Various processes are described in U.S. Pat. Nos. 5,367,040 and 5,637,658 and co-pending, commonly-assigned U.S. application Ser. No. 08/888,878 (filed Jul. 7, 1997), now U.S. Pat. No. 5,756,599, the disclosures of which are incorporated herein by reference.
A principal process for making a consolidated cellulosic product is a "dry" process. In a dry process, filler material, such as cellulosic fibers, is generally coated with a resin binder in a gaseous stream, or by mechanical means. For example, the fibers supplied from a fiberizing apparatus (e.g., a pressurized refiner) may be coated with a thermosetting synthetic resin, such as a phenol-formaldehyde resin, in a blowline blending procedure, wherein the resin is blended with the fiber with the aid of air turbulence. Thereafter, the resin-coated fibers from the blowline are subjected to pre-press drying, for example, in a tube-like dryer, and then are randomly formed into a mat by air conveying the fibers onto a support member (e.g., a forming wire). The formed mat, preferably having a moisture content of less than about 10 wt. %, is then pressed under heat and pressure in a press between a pair of heated platens to cure the thermosetting resin and to compress the mat into an integral consolidated structure. The consolidated structure may be embossed on an outer surface by texturing one of the press platens to achieve a desired embossed design in the outer surface of the product during consolidation.
In a wet-dry process, the resin-blended fiber from the blowline is mixed with water as the conveying medium and is formed into a mat as a wet slurry on a support member where water is removed by mechanical means to a moisture content of about 60% or less. The formed mat is then mechanically conveyed through a multideck air dryer in which the moisture content is further reduced to about 10% or less. The mat is then pressed under heat and pressure similar to the above "dry" process.
Steam injection pressing is a consolidation step that can be used, for example, in dry and wet-dry process production of consolidated cellulosic composites. In steam injection pressing, steam is injected through one or both of the press platens by providing perforations, e.g., drill holes, in the platen(s) so that steam flows into, through, and then out of a mat that includes the synthetic resin and the filler material. The steam condenses on surfaces of the filler and heats the mat. The heat transferred by the steam to the mat, as well as the heat transferred from the press platens to the mat, causes the resin to cure. When compared with conventional pressing operations, steam injection pressing may provide a variety of advantages, such as, for example, shorter press time, a more rapid and satisfactory cure of thicker panels, and products having more uniform physical properties.
Steam injection pressing of mats containing conventional phenolic resins, however, often results in a final composite product exhibiting undesirable characteristics, such as poor bond formation and/or starved glue lines. In order to ensure good bond formation, it is desirable to have a uniform dispersion of the phenolic resin throughout the mat. However, because phenolic resins are water soluble, when mats containing such resins are steam pressed, the steam often condenses and solubilizes the resin. The solubilized resin undesirably migrates to regions of the mat such that the resin is no longer uniformly dispersed, resulting in a product having resin-starved regions and poor bond formation in such regions. Poor bond formation also is attributable to the known phenomena of pre-cure (i.e., where the resin cures before the mat has been compressed to its final dimensions as an integral consolidated structure) and moisture retardation (i.e., where water present in the core or interior of the mat prevents the mat temperature from exceeding the evaporation temperature of water, 100.degree. C., thereby retarding the resin cure). Starved glue lines caused by excess penetration of the resin can occur near the surface of the formed product where resin has undesirably been washed from the surface of the mat and has migrated to edges of the mat or to the mat core. The absence of resin near the product's surface-hence, the presence of starved glue lines-causes flaking of the product.
Another problem encountered with steam injection press methods and apparatus used to consolidate cellulosic products is that it is difficult to achieve complete penetration of steam throughout the full volume of cellulosic material being consolidated in order to achieve uniform heating of the cellulosic material. The following patents teach steam injection through one press platen and exhaustion of the steam through the opposite platen in an attempt to achieve complete flow-through of steam through the cellulosic material being pressed: Shen U.S. Pat. No. 3,891,738; Bottger U.S. Pat. Nos. 4,605,467; and Hsu U.S. Pat. No. 4,850,849. Gawlitta, et al. U.S. Pat. Nos. 5,195,428 and 5,158,012 teach steam injection through parallel channels extending transversely to the length of the platens and steam is exhausted from the opposite side of the platens to achieve complete flow-through of steam. Geimer U.S. Pat. No. 4,393,019 and Taylor U.S. Pat. No. 4,517,147 teach apertured press platens both above and below the cellulosic material being pressed.
Nyberg U.S. Pat. No. 4,162,877; Corbin, et al. U.S. Pat. No. 3,280,237 and Munk et al. U.S. Pat. No. 5,078,938 disclose steam injection from only one press platen and include steam distribution means, such as a multiplicity of small circular openings, e.g., 1-5 mm in diameter. The Corbin '237 patent also teaches the use of a sintered metal plate disposed below the openings, in contact with the mat being pressed, for providing a plurality of minute passages for diffusion of superheated steam, leaving a smoother surface of the finished board. Other patents that disclose the use of steam during hot press consolidation are as follows: Hsu U.S. Pat. Nos. 5,028,286 and 5,134,023; Makinen U.S. Pat. No. 3,686,383 discloses in-situ steam formation from moisture within the cellulosic material and discloses a wire mesh for steam escape; Held U.S. Pat. No. 4,895,508 discloses circular or slot-shaped apertures in pressure bands that are mounted on rollers in continuous cellulosic board formation or through vertically disposed bores adjacent the boards in semi-continuous board production; and Futo U.S. Pat. No. 3,619,450.
As disclosed in the above-mentioned Corbin, et al. '237 patent, another problem encountered with distribution of steam through a cellulosic product being hot press consolidated is that steam distribution apertures adjacent the cellulosic product cause fibers to enter the apertures during the pressing operation, resulting in a bumpy (non-smooth) surface that may require a sanding or other machine-smoothing step. Although not mentioned in the Corbin, et al. '237 patent, while a sintered metal plate provides for excellent steam distribution and provides a smooth surface of the consolidated product, sintered plates provide very slow steam flow rates so that the time required to achieve the required board temperature and steam saturation would be commercially prohibitive, or would result in incomplete steam contact of the cellulosic material being consolidated.
Another problem encountered when the steam distribution apertures are large enough for sufficient steam injection, i.e., the 1-5 mm apertures of the Nyberg 4,162,877 patent, is that cellulosic material that is consolidated within the apertures results in extreme difficulty in separating the product from the apertured steam distribution plate. To achieve a steam injection method and apparatus that achieves sufficient steam distribution throughout the cellulosic material in a commercially acceptable time while achieving a sufficiently smooth surface that is easily separated from a steam distribution plate, therefore, is a formidable task. To achieve sufficient stream distribution in a commercially acceptable time while achieving a smooth surface on the major product face adjacent steam inlets, while achieving a textured or embossed opposite major product face, is even more difficult.
In view of the foregoing, it would be desirable to provide a method of making a cellulosic product that overcomes the problems described above. More particularly, it would be desirable to provide a method of making consolidated cellulosic products, using a conventional resin binder, such as a phenolic resin, and a steam injection pressing operation, that retains the advantages and overcomes the disadvantages of prior methods of making cellulosic composites.